Image system with non-circular patient aperture

ABSTRACT

A radiographic imaging system ( 10 ) includes an array of detectors ( 16 ) arranged around a circular bore ( 18 ). A subject ( 14 ) is injected with a radioisotope. The subject ( 14 ), supported by a subject support means ( 12 ), is to be placed into the bore ( 18 ) for an examination. A fixed, non-circular shield ( 38 ) is rigidly mounted to one of an entrance ( 40 ) and an exit ( 42 ) of the bore ( 18 ) to prevent emission radiation originating outside of the bore ( 18 ) to reach the radiation detectors ( 16 ). The shield ( 38 ) extends from an outer periphery of the bore ( 18 ) toward and surrounding a central axis of the bore ( 18 ) and defines a fixed, non-circular subject receiving aperture ( 36 ). The shield ( 38 ) is tailored to a contour of the subject support means ( 12 ) and the subject ( 14 ) to permit the subject of a maximum girth to be received in the fixed, non-circular aperture ( 36 ).

The present invention relates to the diagnostic imaging systems andmethods. It finds particular application in conjunction with thePositron Emission Tomography (PET) scanners and will be described withparticular reference thereto. It will be appreciated that the inventionis also applicable to other radiological scanners and the like.

PET is a valuable patient imaging scanner employing positron emittingcompounds. PET provides specific metabolic information about tissuesthat conventional scanners such as CT and MRI can not provide.Typically, PET scanners include a circular bore that is surrounded by acircular array of detectors which detect concurrent energy events. Priorto the scan, the patient is injected with a positron emittingradioisotope which is taken up by cells. When a positron emits from aradioisotope, it combines with an electron to produce an annihilationreaction, in which the pair's mass is converted into energy. The energyis dispersed in the form of two 511 kev gamma rays or photons, traveling180 degrees apart. When two detectors “see” 511 kev photons from theannihilation event concurrently or within nanoseconds of each other, thedetectors register a coincidence along the line between the detectorpoints—a line of response (LOR). The PET system draws lines of responsesbetween each detection pair, registering coincidence events during thescan. When the scan is completed, there are areas of overlapping linesthat indicate more concentrated areas of radioactivity. The system usesthis information to reconstruct a three dimensional image ofradioisotope concentration in the body.

The scanner accepts photons from anywhere from the field of view, and,in addition, accepts photons originating outside of field of view thatinto travel into the field of view. The photons originating outside ofthe field of view do not contain useful information that is used inimage reconstruction. Note that one of the 180° opposite photons for apoint outside the field of view normally cannot strike the detector.Typically, the PET systems are designed to only accept coincidenceevents within a narrow window. Problems arise when trying to run thescanner in a high flux photon situation where the total activity seen bythe system is high. The higher the number of photons detected per unittime, the higher the probability that not-paired photons will bedetected within the coincidence time window. The systems performancedegrades with high activity levels in patient bodies. This becomes anissue when the patient is injected with the radioisotopes having a shorthalf life which is not enough to obtain sufficient valid counts becauseof detection errors attributable to high flux. By shielding thedetectors from out of the field of view events, the total flux acrossthe system can be lowered while not reducing the useful truecoincidences from the target organ occurring in the window ofacceptance. Having lowered the total flux, more tracings can be usedresulting in a higher number of true coincidence events being recordedfrom the organ of interest in the window of acceptance for a particularamount of the radioisotope injected.

Typically, the shield is a lead flange at the entrance and exit of thePET scanner bore. The flange extends from the outer periphery of thebore toward the central axis of the bore and leaves a circular patientaperture of about 50-60 cm in diameter. Although it is important toshield as much as possible, the smaller opening presents a problem whenthe larger patients do not fit through it, while the larger opening doesnot provide an effective shielding.

There is a need for a shielding for the PET scanners that betterconforms to the patient's contour and provides effective shielding, yetis inexpensive and easy to handle. The present invention provides a newand improved imaging apparatus and method which overcomes theabove-referenced problems and others.

In accordance with one aspect of the present invention, a radiographicimaging system is disclosed. A means detects emission radiation emittedby a radioisotope injected into a subject, the detecting means arrangedaround a circular bore, which has an entrance and an exit. A meansshields the detecting means from the emission radiation originatingoutside of the bore. The shielding means includes at least one rigidradiation opaque shield rigidly mounted to one of the entrance and theexit of the bore. The shield extends from an outer periphery of the boretoward and surrounding a central axis of the bore and defining a fixednon-circular subject receiving aperture.

In accordance with another aspect of the present invention, a method ofradiographic imaging is disclosed. Emission radiation, which is emittedby a radioisotope injected into a subject, is detected along a detectingmeans defined around a circularly cylindrical bore. The detecting meansis shielded from the emission radiation originating outside of the borewith at least one shield rigidly mounted to one of an entrance and anexit of the bore and extending from an outer periphery of the boretoward and surrounding a central axis of the bore. The at least oneshield defines a fixed non-circular subject receiving aperture.

One advantage of the present invention resides in reducing out-of-the-field-of-view events reaching the detectors.

Another advantage of the present invention resides in fitting radiationshielding to the patient, yet providing an easy to maintain andinexpensive shielding.

Another advantage of the present invention resides in its mechanicalsimplicity and lack of moving parts.

Still further advantages and benefits of the present invention willbecome apparent to those of ordinary skill in the art upon reading andunderstanding the following detailed description of the preferredembodiments.

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot be construed as limiting the invention.

FIG. 1 is a diagrammatic illustration of a PET imaging system inaccordance with present invention;

FIG. 2 is a diagrammatic illustration of a non-circular subjectreceiving aperture in a shape of an ellipse;

FIG. 3 is a diagrammatic illustration of a non-circular subjectreceiving aperture tailored to the couch.

With reference to FIG. 1, an imaging system 10 includes a subjectsupport means 12, such as a table or couch, which supports a subject 14being imaged. The subject 14 is injected with one or more radioisotopesto induce the emission of the positron. An annular array of detectors 16is arranged around a circular bore 18. Because the detectors may haveplanar faces the detector array 16 may be an octagon or other regularpolygon that approximates a circle. The subject support 12 is advancedand retracted to achieve the desired positioning of the subject 14within the bore 18, e.g. with the region of interest centered in thefield of view of the detectors. Radiation events detected by detectors16 are collected by a line of response (LOR) calculating circuit 20. TheLOR calculator 20 includes a coincidence detector 22 that determineswhen two events are within a preselected temporal window of beingsimultaneous. From the position of the detectors 16 and the positionwithin each detector, at which the coincident radiation was received, aray between the radiation detection points is calculated by line extrapolator 24.

The acquired LOR data are preferably stored in a data memory or buffer26. A data reconstruction processor 28 reconstructs an electronic imagerepresentation from the LOR data stored in data memory 26 and stores theresultant image representation in an image memory 30. Portions of thestored image representation are retrieved by an image processor 32 andconverted to an appropriate format for display on a monitor 34, such asa video, CCD, active matrix, or other monitor. Of course, a colorprinter or other output device may also be used to present the data in aconvenient format.

With continuing reference to FIG. 1 and further reference to FIG. 2, anon-circular receiving area or aperture 36 of the PET scanner is definedby radiation shields 38 mounted at the entrance and exit of the circularbore 18 and extending from an outer periphery of the bore 18. Theshields 38 are manufactured from a LEAD or other high density shieldingmaterial and is up to 25 mm thick. The aperture 36 has a largerdimension D1 along the axis substantially parallel to the horizontal,transverse axis drawn through the shorter dimension of the couch 12. Inthe preferred embodiment, the aperture 36 is an ellipse, with the largerdimension or major axis D1 equal to 70 cm. The aperture 36 has a smallerdimension or minor axis D2 along the vertical axis perpendicular to theaxis drawn through the shorter dimension of the couch 12. In thepreferred embodiment, the smaller dimension D2 of oval aperture 36 is 50cm. More specifically, aperture is sized such that a nominally sizedsubject centered in the aperture is generally equidistant from theshield in all directions.

With reference to the embodiment of FIG. 3, the PET scanner has aradiation shield 38 mounted at the entrance 40 and exit 42 of thecircular bore 18, which defines a non-circular aperture 36. A bottomboundary 50 of aperture 36 is disposed underneath the couch 12 andfollows closely the shape of a bottom surface 52 of the couch 12. Whenthe couch 12 is in a lower most position, there is no substantial airgap between the aperture bottom boundary 50 and the bottom surface 52 ofcouch 12. A top boundary 54 of aperture 36 formed by shield 38 ispositioned above the subject 14. Preferably, it is curved to allow thesubject 14 of maximum girth to be received in the subject receivingaperture 36 with the couch 12 in the lower most position. Sideboundaries or sides 56 are formed between the top and bottom boundariesof aperture 36. The sides 56 of the aperture 36 are linear verticalsegments, which are closely adjacent the sides 58 of couch 12 and extenda vertical distance commensurate with the permitted vertical travel ofthe couch 12. Preferably, the sides 56 transition into the top boundary54 with curved surfaces that conform to an upper side contour of thesubject 14. Preferably, there is no substantial air gap between theaperture sides 56 and the sides 58 of couch 12 as well as between theaperture sides 56 and the sides of a maximum width of subject 14.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others upon areading and understanding of the preceding detailed description. It isintended that the invention be constructed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. A radiographic imaging system comprising: a means for detectingemission radiation emitted by a radioisotope injected into a subject,the detecting means arranged around a circular bore, the bore having anentrance and an exit; and a means for shielding the detecting means fromthe emission radiation originating outside of the bore, the shieldingmeans including at least one rigid radiation opaque shield rigidlymounted to one of the entrance and the exit of the bore, the shieldextending from an outer periphery of the bore toward and surrounding acentral axis of the bore and defining a fixed non-circular subjectreceiving apertures.
 2. The imaging system according to claim 1, whereinthe aperture of the at least one shield is elliptical.
 3. The imagingsystem according to claim 2, wherein the elliptical aperture has ahorizontal major axis and a vertical minor axis.
 4. The imaging systemaccording to claim 3, wherein a ratio of the major axis to the minoraxis is or about 7 to
 5. 5. The imaging system according to claim 1,wherein the subject is received in the aperture on a subject supportmeans including: a top surfaced, on which the subject is positioned; abottom surface opposing the top surface; and a pair of side surfacesopposing each other and each disposed between the bottom and the topsurfaces.
 6. The imaging system according to claim 5, wherein eachshield defines a bottom boundary of the aperture disposed underneath thesubject support means, which bottom boundary of the aperture conforms toa shape of the bottom surface of the subject support means.
 7. Theimaging system according to claim 6, wherein the subject support meansis vertically adjustable and no substantial air gap is defined betweenthe aperture bottom boundary and the bottom surface of the subjectsupport means when the subject support means is in a lower mostposition.
 8. The imaging system according to claim 6, wherein eachshield defines a top curved boundary of the aperture disposed above thesubject support means.
 9. The imaging system according to claim 8,wherein each shield defines a pair of opposing side boundaries of theaperture, each side boundary disposed between the bottom and topboundaries of the apertures.
 10. The imaging system according to claim9, wherein the aperture side boundaries are curved.
 11. The imagingsystem according to claim 9, wherein the aperture side boundariesinclude linear vertical surfaces, which conform to a path of verticaltravel of the side surfaces of the subject support means and thesubject.
 12. The imaging system according to claim 11, wherein there isno substantial air gap between each side boundary of the aperture and anassociated side surface of the subject support means.
 13. The imagingsystem according to claim 11, wherein there is no substantial air gapbetween side boundaries of the aperture and the subject.
 14. The imagingsystem according to claim 9, wherein: the bottom boundary of theaperture is substantially parallel to the bottom surface of the subjectsupport means, and each side boundary of the aperture is substantiallyparallel to an associated side surface of the subject support means. 15.The imaging system according to claim 1, wherein: at least one of theshields is a plate of radiation opaque material which is non-movablymounted about the bore.
 16. The imaging system according to claim 1,wherein the emission radiation detecting means includes a plurality ofdetectors mounted around the circular bore and further including: acoincidence detecting means for determining when two of the detectorsdetect emitted radiation within a preselected temporal window of beingsimultaneous.
 17. A method of radiographic imaging comprising: detectingemission radiation emitted by a radioisotope injected into a subjectalong a detecting means defined around a circularly cylindrical; andshielding the detecting means from the emission radiation originatingoutside of the bore with at least one shield rigidly mounted to one ofan entrance and an exit of the bore and extending from an outerperiphery of the bore toward and surrounding a central axis of the boreand defining a fixed non-circular subject receiving aperture.
 18. Amethod of shielding a radiographic scanner, which has an elongatedcircular bore extending between first and second bore ends andsurrounded by an array of radiation detectors, from radiationoriginating outside of the bore, the method comprising: shaping aunitary piece of radiation opaque material into a one-piece shield withan outer periphery that closes one of the bore ends and a centralnon-circular aperture which mimics a cross section of a receivedsubject; and rigidly mounting the shield to the one bore end to permit asubject to be imaged in the scanner bore to pass into and out of thebore through the non-circular aperture.
 19. The method according toclaim 18, wherein the aperture is elliptical.
 20. The method accordingto claim 18, wherein the radiographic scanner includes a subjectsupport, which supports the subject in the bore and moves the subjectlongitudinally into and out of the bore through the non-circularaperture, the method further including: shaping a bottom boundary of thenon-circular aperture to conform to a shape of a bottom surface of thesubject support.
 21. The method according to claim 20, furtherincluding: shaping a top boundary of the aperture disposed above thesubject support means arcuately with a different curvature from thebottom boundary.
 22. The method according to claim 20, wherein thesubject support moves vertically to raise and lower the subject in thebore and further including: shaping a pair of opposing side boundariesof the aperture with linear and vertical regions to accommodate verticalmovement of the subject support.
 23. The method according to claim 18,further including: positioning a subject on a subject support; injectingthe subject with a radiopharmaceutical; moving the subject support toposition a region of interest of the subject in an isocenter of the boreand other regions of the subject outside of the bore; and detectingradiation from the radiopharmaceutical within the region of interestwith the array of radiation detectors, while concurrently blockingradiation from the radiopharmaceutical in the regions of the subjectoutside the bore from reaching the radiation detectors with theradiation opaque shield.
 24. A diagnostic imaging system comprising: aplurality of emission radiation detectors arranged to define an imagingregion; and a radiation shield positioned at least one end of theimaging region, wherein said radiation shield includes a non-circularsubject receiving aperture.